Titanium carbide-nickel compositions

ABSTRACT

DENSE, HOMOGENEOUS COMPOSITIONS HAVING (A) A DENSITY IN EXCESS OF 95% OF THEORETICAL AND (B) AN AVERAGE GRAIN SIZE LESS THAN 10 MICRONS AND CONSISTING OF:   (1) 80-97 PERCENT BY VOLUME OF TITANIUM CARBIDE AND (2) 3-20 PERCENT BY VOLUME OF NICKEL, THE NICKEL BEING PRESENT MAINLY AS A BINDER PHASE UNIFORMLY DISTRIBUTED AT THE GRAIN BOUNDARIES, ARE USEFUL AS CUTTING EDGES FOR CUTTING TOOLS.

nited S tes Patented July 4, 1972 3,674,443 TITANIUM CARBIDE-NICKELCOMPOSITIONS Alma U. Daniels, Wilmington, Del., assiguor to E. I. duPont de Nemours and Company, Wilmington, Del. No Drawing. Filed May 28,1969, Ser. No. 828,699 Int. Cl. B22f 3/ 14 US. Cl. 29182.8 4 ClaimsABSTRACT OF THE DISCLOSURE Dense, homogeneous compositions having (a) adensity in excess of 95% of theoretical and (b) an average grain sizeless than 10 microns and consisting of:

( 1) 80-97 percent by volume of titanium carbide and (2) 3-2 percent byvolume of nickel, the nickel being present mainly as a binder phaseuniformly distributed at the grain boundaries, are useful as cuttingedges for cutting tools.

BACKGROUND In the past, many attempts have been made to make strongdense compositions which have small average grain sizes by sinteringtitanium carbide and nickel compositions. Such titanium carbide-nickelbodies have very large average grain sizes and the nickel has not beenuniformly distributed, possibly due to inadequate wetting of thetitanium carbide by the nickel, and furthermore the bodies have beenlacking in strength. To overcome this problem, it has been the practiceto use molybdenum along with the nickel. See Swinehart, Cutting ToolMaterials Selection A.S.T.M.E., Dearborn, page 89 (1963). The use ofmolybdenum along with the nickel gives improved wetting of the carbidephase and enables a strong sintered composition to be made.

It has now been discovered that strong, dense titanium carbide-nickelbodies can be prepared in the absence of molybdenum when the process astaught in the instant invention is employed.

SUMMARY OF THE INVENTION It has been found that dense titaniumcarbide-nickel bodies with average grain sizes smaller than microns canbe prepared when particular process conditions are used.

Thus, the invention relates to the following compositions of matter:homogeneous compositions having (a) a density in excess of 95% oftheoretical and (b) an average grain size less than 10 micronsconsisting of:

(1) 80-97 percent by volume of titanium carbide and (2) 3-20 percent byvolume of nickel, the nickel being present mainly as a binder phaseuniformly distributed at the grain boundaries, substantially all of saidbinder phase having a thickness of less than 0.5 micron.

These compositions demonstrate exceptional advantages over sinteredtitanium carbide-nickel compositions and as a result of theirexceptional properties the compositions of this invention are useful ascutting tools in machining materials such as cast iron and alloy steel.

The above compositions can be made by:

(I) Intimately milling together a mixture of fine titanium carbide andnickel powder in a hydrocarbon oil and separating, drying and screeningthe powder in a nonoxidizing atmosphere;

(II) Charging in a non-oxidizing atmosphere the titanium carbide-nickelpowder to a graphite mold provided with close fitting graphite pistonsand in either sequnce (a) applying to the power by means of said pistonsa pressure of 0-500 p.s.i. and (b) introducing the mold into a vacuumfurnace preheated to a temperature in the range of from 1000 C. to 1475C.;

(III) Heating said mixture under vacuum to a selected temperature in therange of from 1350" C. to 1475' C. in a period of time ranging fromabout 2 minutes to 20 minutes while maintaining a pressure of from 0-500p.s.i.;

(IV) Maintaining the final temperature and pressure of Step III for aperiod of time ranging from zero minutes to 5 minutes;

(V) Maintaining the temperature of Step III and applying to the sampleby means of the graphite pistons a pressure in the range of from 500p.s.i. to 4000 p.s.i. for a period of time ranging from 2 minutes to 6minutes; and

(VI) Thereafter immediately releasing the pressure on the pistons,removing the die from the furnace and rapidly cooling the resultingtitanium carbide-nickel composition.

DETAILED DESCRIPTION Starting materials Titanium carbide is used in thecompositions of this invention in amounts ranging from -97 percent byvolume. At least 80 volume percent titanium carbide is required toprovide adequate hardness. Preferred amounts of carbide range from -95percent by volume to achieve an optimum balance of strength andhardness.

Titanium carbide can be obtained commercially or can be synthesized bymethods well known to the art. The titanium carbide should be finelydivided, that is it should have a particle size of less than 5 micronsand preferably less than 2 microns. If the starting material isappreciably larger than 5 microns in particle size it can be pre-groundto reduce its size to that which is acceptable. Of course the milling ofthe components of this invention, which is carried out to obtain a highdegree of homogeneity, will result in some comminution of the carbideand the other starting component, nickel.

Nickel is used in the compositions of this invention in amounts rangingfrom 3-20 percent by volume. At least 3 volume percent of nickel isrequired to provide adequate strength. The preferred amounts of nickelare 5-5 percent by volume since an optimum balance of strength andhardness are achieved in this range.

The starting materials used in the compositions of this invention shouldbe pure. In particular it is desired to exclude impurities such asoxygen which tends to have deleterious effects on the densecompositions. On the other hand minor amounts of many impurities can betolerated with no appreciable loss of properties. Thus the metal cancontain small amounts of other metals such as titanium, zirconium,tantalum or niobium as minor impurities, although low melting metalslike lead should be excluded. Small amounts of carbides other thantitanium carbide, such as several percent of tungsten carbide, which issometimes picked up in grinding, can be present. Even oxygen can betolerated in small amounts such as occurs when titanium carbide has beenexposed to air resulting in a few percent of titanium oxy-carbide.However, after the powder components have been milled together and arein a highly reactive state, oxidation, particularly of the metals,occurs easily and should be avoided.

PROCESS OF MANUFACTURE The manner in which the compositions of thisinvention are prepared is important, because the characteristics of thecompositions are achieved in part at least as a result of the manner inwhich they are prepared. Thus the use of fine-grained starting materialsand thorough milling of the mixed components are directly related to thefine grain size and uniform homogeneity of the compositions. Otherprecautions which should be observed in preparing the compositions whichhave important effects on the products are:

1) The prevention of excessive contamination from grinding media andmoisture or oxygen in the air;

(2) Hot-pressing or presintering under conditions which permit theescape of volatile materials prior to densification;

(3) Avoiding undue absorption of carbon from pressing molds by limitingtheir contact under absorption-promoting conditions;

(4) Avoiding excessive component recrystallization and resultantsegregation by avoiding prolonged subjection to very high temperatures.

(1) Milling and powder recovery Milling of the components, tohomogeneously intermix them and obtain very fine grain sizes is carriedout according to the practices common in the art. If roller milling isemployed, optimum milling conditions will ordinarily involve a millhalf-filled with a grinding medium such as cobalt bonded tungstencarbide or alumina balls or rods, a liquid medium such as a hydrocarbonoil, an inert atmosphere, grinding periods of from a few days to severalweeks, and powder recovery also in an inert atmosphere. The recoveredpowder is ordinarily dried at temperatures of around ISO-300 C. undervacuum, followed by screening and storage when desirable in an inertatmosphere. Other methods of milling, such as vibratory milling can alsobe used in preparing the powders.

(2) Consolidation The compositions of this invention are ordinarilyconsolidated to dense, pore-free bodies by sintering under pressure.Consolidation is ordinarily carried out by hot pressing the mixedpowders in a graphite mold under vacuum.

The hot pressing process consists of loading milled and dried powderinto a graphite mold or die provided with close fitting graphite pistonsand subjecting it to a pressure of -500 p.s.i., applied to the pistonsby means of hydraulically activated rams, inserting the mold into theheated zone (1000 C.-1475 C.) of a vacuum hot press, thus allowingvolatile impurities to escape before the composition is densified. Fullpressure (500-4000 p.s.i.) is then applied to the sample at or nearmaximum temperature (1350l475 C.) employed for a period of time rangingfrom 2 minutes to 6 minutes.

The pressure is then released and the mold containing the densified bodyis immediately ejected from the heated zone of the hot press and cooledrapidly in the course of a few minutes to dull red heat while stillunder vacuum.

Maximum or goal temperatures for hot pressing range between 1350 C. and1475 C. depending on the amount of nickel which is present. Thetemperatures will ordinarily be about 1400 C. Full pressures used duringhot pressing ordinarily range between 500 and 4000 p.s.i., with lowpressures being used in combination with lower temperatures forcompositions with a high nickel content. Conversely, higher pressuresand temperatures are employed for compositions low in nickel.

As would be expected, at higher temperatures and pressures, some of thenickel may tend to squeeze out of the compositions when pressure isapplied. This tendency can be compensated for by starting with a littlemore nickel than is desired in the final body when operating at a hightemperature and pressure. By this procedure, some of the nickel will besqueezed out during pressing, leaving the body with the desired metalcontent. Generally speaking, appreciable squeeze-out of metal is to beavoided not only because it changes the composition but also because themetal causes sticking to and damaging of the molds.

It is important that during hot pressing, the compositions not be heatedto a goal temperature for a period of time which is much in excess ofthat required to eliminate porosity and achieve density. Such highertemperatures or longer times can result in excessive grain growth,coarsening of the structure, the development of secondary porosity dueto recrystallization, or in the formation of undesirable phases.

The products of this invention are ordinarily subjected to pressure atmaximum temperature for 2-6 minutes after which the product isimmediately removed from the hot zone. The resultant bodies are finegrained, homogeneous, essentially pore-free, and are characterized byhigh hardness and excellent transverse rupture strength.

CHARACTERISTICS OF THE COMPOSITIONS In addition to characterizing thecompositions of this invention on the basis of the components discussedabove, the compositions can also be characterized on the basis of theirstructural characteristics, i.e., fine grain size and homogeneity.

The dense bodies of this invention are characterized as having a fineaverage grain size smaller than 10 microns and preferably smaller than 5microns in average grain diameter. Moreover, the grain size is uniformthroughout the compositions and there is essentially no porosity in thedense compositions of this invention. The fine grain size and lowporosity of dense compositions of this invention contribute greatly toits hardness and thus result in bodies which are exceptionallyabrasion-resistant. For example, cutting tools made from dense bodies ofthis invention resist abrasion when coming in contact with the hardcarbide inclusions that are found in cast iron.

Distribution of the titanium carbide and nickel in dense bodies of thisinvention is uniform and homogeneous, and, generally speaking, any area100 microns square which is examined microscopically at l000magnification will appear the same as any other area 100 microns squarewithin conventional statistical distribution limits. The nickel ispresent mainly as a binder phase uniformly distributed at the grainboundaries, substantially all of said binder having a thickness of lessthan 0.5 micron, in contrast to compositions prepared by conventionalsintering where the nickel is largely present as heterogeneouslydistributed inclusions in a coarse-grained weak body. The combination offine grain size and homogeneity of distribution of the components indense bodies of this invention results in bodies which are resistant tothermal shock both as regards shattering and as regards surfaceheat-cracking.

The products may contain small amounts of iron, cobalt, tungstencarbide, alumina, and other impurities, which are generally picked upduring the milling process. When metallic impurities are present, theywill generally constitute less than about 1% by volume of the totalcomposition. When alumina inserts are used during milling, for examplein a vibratory mill, there may be as much as 5% by volume of aluminapresent.

The products of this invention will have actual densities in excess ofof theoretical, preferably in excess of 98% of theoretical. Theoreticaldensity for any given composition is determined by the followingequation:

p =4.95V 1c+8.90V 1 wherein:

=theoretical density in grams per cubic centimeter;

V =desired volume in cubic centimeters of titanium carbide per cubiccentimeter of product;

V =desired volume in cubic centimeters of nickel per cubic centimeter ofproduct.

A comparison of actual density to theoretical density is of course anindication of the degree of porosity.

EXAMPLES The invention will be better understood in reference to thefollowing examples wherein parts are by weight unless otherwise noted.

Example 1 This is an example of a composition containing 93 volumepercent titanium carbide and 7 volume percent nickel.

The titanium carbide used is a fine powder with a specific surface areaof 3 m. /g., as determined by nitrogen adsorption. An electronmicrographs shows that the titanium carbide grains are approximately 2microns in diameter, and are clustered in the form of loose aggregates.The carbon content is 19.0% and the oxygen analysis indicates a titaniumdioxide content of about 2.5%.

The nickel used is a fine powder containing 0.15% carbon, 0.07% oxygen,and less than 300 parts per million iron. The specific surface area ofthe nickel powder is 0.48 m. g. and its X-ray diffraction pattern showsonly nickel which has a crystallite size of 150 millimicrons ascalculated from line broadening. Under the electron microscope, thepowder appears as polycrystalline grains, 1 to microns in diameter.

The powders are milled by loading 8300 grams of cylindrical 6%cobalt-bonded tungsten carbide inserts, A" long and A in diameter in a1.8 liter steel rolling mill that is completely lined with 6%cobalt-bonded tungsten carbide. The inserts have been previously worn inso that contamination of powder batches with tungsten carbidecobalt willbe kept to a few percent. The mill is charged with a mixture of 500 ml.of Soltrol 130 (a saturated parafiinic hydrocarbon with a boiling pointof approximately 130 C.), 230 grams of the titanium carbide and 31.2grams of the nickel as above described.

The mill is then sealed and rotated at 90 r.p.m. for 5 days. The mill isthen opened and the contents emptied while keeping the miling insertsinside. The mill is then rinsed out with Soltrol 130 several times untilessentially all of the milled solids are removed.

The milled powder and liquid are then transferred to a vacuum evaporatorand the excess hydrocarbon is decanted off after the suspended materialhas settled. The wet residual cake is then dried under vacuum with theapplication of heat until the temperature within the evaporator isbetween 200 and 300 C., and the pressure is less than about 0.1 ml. ofmercury. Thereafter, the powder is handled entirely in the absence ofair.

The dry powder is passed through a 70 mesh screen in a nitrogenatmosphere, and then stored under nitrogen and sealed in plasticcontainers.

A consolidated billet is prepared from this powder by hot pressing thepowder in a cylindrical gra hite mold having a cylindrical cavity 1 indiameter and which is equipped with opposingclose-fitting graphitepistons. One piston is held in place in one end of the mold cavity,while 22 grams of the powder is dropped into the cavity under nitrogenand evenly distributed by rotating the mold and tapping it lightly onthe side. The upper piston is then put in place under hand pressure. Theassembled mold and contents are then placed in the vacuum chamber of avacuum hot press. The mold is held in a vertical position and thepistons extending above and below are engaged between opposite graphitegrams of the press under pressure of about 100 p.s.i. Within a period ofa minute, the mold is raised into the hot zone of the furnace at 1000C., and at once the temperature of the furnace is increased while thepressure is maintained at 100 p.s.i. during the heat-up period. Thetemperature is raised from 1000 C. to 1400 C. in about 4 minutes, andthe temperature of the mold is then held at 1400 C. for another 2minutes to ensure uniform heating of the sample. A pressure of 4000p.s.i. is applied to the billet through the pistons for 4 minutes. Thepressure is then released and the mold and contents are immediatelymoved out of the furnace into a cool zone and cooled to dull red heat inabout 5 minutes.

After further cooling, the mold and contents are removed from the vacuumfurnace and the billet is removed from the mold and sand-blasted toremove any adhering carbon.

The hot pressed billet is found to be essentially nonporous, having novisible porosity under 1000 magnification. An electron micrograph at20,000 magnification on a sample etched with a mixture of hydrofluoric,nitric and sulfuric acids shows a homogeneous composition having anaverage grain size of about one micron, with a nickel-rich binder phaseuniformly distributed at the grain boundaries and having a thickness of0.05 to 0.3 micron. Few of the grains exceed 2 or 3 microns in size. Thedensity is 99% of the theoretical density.

X-ray diffraction analysis reveals primarily a sharp, strongface-centered cubic type pattern with atomic spacing of 4.323 angstromsindicating titanium carbide, and also diffraction lines showing thepresence of free nickel.

Chemical analysis shows, in addition to titanium, carbon and nickel, thepresence of about 2.7% of tungsten and 0.15 of cobalt. The tungsten andcobalt are presumably picked up from attrition of the milling insertsand mill lining. The billet, which is 1" in diameter, and about 0.30" inthickness, is cut so that a piece slightly larger than /2 square isremoved from the center. Strips 0.070" in thickness are cut from thematerial remaining to each side of the center piece, and are further cutinto 0.070" x 0.070" square bars, for testing transverse rupturestrength. Other portions of the billet are used for indentation hardnesstests and for other product characterizations. The transverse rupturestrength as measured by breaking the 0.070" x 0.070" test bars on a spanis about 211,000 p.s.i. The hardness of the billet is 93.0 on theRockwell A scale.

The square center piece is finished as a cutting tip to /2" x /2" x 7and the corners are finished with a radius, a style known in theindustry as SNG-432.

The tip is used to turn a cylinder of Class 30 grey cast iron at a speedof 1250 surface feet per minute, a depth of 0.050" and a feed of 0.005"per revolution. After ten minutes the Wear land on the flank of thecutting edge is only 0.006" deep, which is only half the flank wearmeasured on a commercial titanium carbide based cutting tip after tenminutes turning under the same conditions. When used to mill 4340 steelhaving a Brinnell hardness number of 340 at 1000 surface feet perminute, a width of cut of 2", a depth of 0.050" and a feed of 0.0057"per revolution, for a single tooth 6" diameter milling head, two cornersof the insert run an average distance of 24" before failing by chipping.Two corners of a commercial titanium carbide based cutting tip testedunder the same condition give an average cutting distance of only 16"'before failing due to chipping. Thus the composition of the inventionshows a considerable improvement over conventional titanium carbidebased compositions, both with respect to Wear and chip resistance inmachining operations on two important classes of metal.

EXAMPLE 2 The procedure of Example 1 is repeated, except that theproportions of the components are changed to give a powder containing 86volume percent of titanium carbide and 14 volume percent of nickel.

A dense body made by hot pressing this powder as described in Example 1is found to have a transverse bend strength of 207,000 p.s.i. and aRockwell A hardness of 92.2. Examination of an electron micrograph at amagnification of 20,000 on a sample etched as in Example 1 shows ahomogeneous composition having an average grain size of about 0.8 micronand a nickel-rich binder phase uniformly distributed at the grainboundaries and having a thickness of 0.1 to 0.5 micron. The density is98.5% of the theoretical density.

An SNG-432 cutting tip made from the hot pressed body is found to yieldexcellent results in the semi-finishing turning of Class 30 gray castiron and in the finish milling of Class 30 gray cast iron.

Example 3 The procedure of Example 1 is used, changing the proportionsof the components to give a powder containing 97 volume percent oftitanium carbide and 3 volume percent of nickel. The powder is hotpressed as described in Example 1, modifying the conditions as follows.The sample in the mold is pressed at 500 p.s.i., before being introducedinto the hot zone of the vacuum hot press at 1200 C. It is brought to1475 C. maintaining the pressure on the sample at 500 p.s.i. during theheat-up period of about three minutes. After holding at 1475 C. for fiveminutes to ensure uniform heating of the sample, the pressure isincreased from 500 p.s.i. to 40-00 p.s.i. and this pressure andtemperature of 1475 C. are maintained for a further six minutes. Thepressure is then released and the mold and contents are immediatelymoved out of the furnace into a cool zone and cooled to dull red heat inabout five minutes.

The resulting dense body is found to have a transverse rupture strengthof 155,000 p.s.i. and a Rockwell A hardness of 93.5. Electron micrographexamination of a sample etched as in Example 1 shows a homogeneouscomposition having an average grain size of about three microns and anickel-rich binder phase uniformly distributed at the grain boundariesand having an average thickness of 0.02 to 0.2 micron. The density is97% of the theoretical denesity.

A seal vane in a centrifugal pump for pickling acid in a steel plant ismade from the above composition, and after being in service for sixmonths, inspection of the part shows very little wear or corrosiveattack and the part is returned to service.

UTILITY The pressed products show excellent performance as cutting toolsand have advantages over conventional TiC-based tools in certainapplications such as turning cast iron and milling alloy steel. Theproducts can also be used as wear parts and refractory and corrosionresistant parts in high temperature equipment.

I claim:

1. A homogeneous composition consisting of to 97% by volume of titaniumcarbide and 3 to 20% by volume of nickel and having (a) an average grainsize of less than 10 microns, (b) a binder phase consisting essentiallyof nickel uniformly distributed at the grain boundaries, substantiallyall of said binder phase having a thickness of less than 0.5 micron, and(c) a density in excess of 95% of the theoretical density.

2. A composition of claim 1 having a density in excess of 98% of thetheoretical density.

3. A composition of claim 2 consisting of to by volume of titaniumcarbide and 5 to 15% by volume of nickel.

4. A composition of claim 1 wherein the average grain size is less than5 microns.

References Cited UNITED STATES PATENTS 2,752,666 7/1956 Goetzel et al.29182.8 2,714,245 8/1955 Goetzel 29182.8

FOREIGN PATENTS 961,181 6/1964 Great Britain. 406,633 7/ 1932 GreatBritain.

OTHER REFERENCES Swarzkopf et al.: Refractory Hand Metals, 1953, Mac-Millan Company, pp. 376-7.

Jones: Fundamental Principles of Powder Metallurgy, 1960, Edward ArnoldLtd., pp. 4867.

CARL D. QUARFORTH, Primary Examiner B. H. HUNT, Assistant Examiner U.S.Cl. X.R.

